rope_htmlrender: fix some HTML rendering bugs

[ee7]

Fixes crashappsec/chalk#178.

Consider this HTML:

<table>
  <caption>Commands</caption>
  <tbody>
    <tr>
      <th>Command Name</th>
      <th>Description</th>
    </tr>
    <tr>
      <td>insert</td>
      <td>Add chalk marks to artifacts</td>
    </tr>
    <tr>
      <td>docgen</td>
      <td>Generate technical documentation</td>
    </tr>
  </tbody>
</table>

Before this PR, if we converted that to a Rope then called toHtml, we'd get invalid HTML:

<div>

<table>

<tbody>

<td>

<th>
Command Name</th>
</td>


<td>

<th>
Description</th>
</td>


<td>

<td>
insert</td>
</td>


<td>

<td>
Add chalk marks to artifacts</td>
</td>


<td>

<td>
docgen</td>
</td>


<td>

<td>
Generate technical documentation</td>
</td>
</tbody>

<caption>

<caption>
Commands</caption>
</caption>
</table>

<div>
</div>
</div>

The errors:

• an extra <td> pair around each cell
• no <tr> anywhere
• <caption> at the bottom, but <caption> must be the first child of <table>

See the errors when pasting into https://validator.w3.org/#validate_by_input.

With this PR, toHtml produces valid HTML for that input:

<div>
<table>
<caption>
Commands
</caption>

<tbody>

<tr>

<th>
Command Name
</th>

<th>
Description
</th>

</tr>

<tr>

<td>
insert
</td>

<td>
Add chalk marks to artifacts
</td>

</tr>

<tr>

<td>
docgen
</td>

<td>
Generate technical documentation
</td>

</tr>

</tbody>
</table>
<div>

</div>

</div>

which prettifies to something that's the same as the input, except:

+<div>
   <table>
     <caption>Commands</caption>
     <tbody>
       <tr>
         <th>Command Name</th>
         <th>Description</th>
       </tr>
       <tr>
         <td>insert</td>
         <td>Add chalk marks to artifacts</td>
       </tr>
       <tr>
         <td>docgen</td>
         <td>Generate technical documentation</td>
       </tr>
     </tbody>
   </table>
+  <div></div>
+</div>